Process for producing monolithic film of integrated highly oriented halogenated graphene sheets or molecules

ABSTRACT

A process for producing an integrated layer (10 nm to 500 μm) of highly oriented halogenated graphene sheets, comprising: (a) preparing a graphene oxide (GO) dispersion having GO sheets dispersed in a fluid medium; (b) dispensing and depositing a layer of GO dispersion onto a surface of a supporting substrate under a shear stress condition that induces orientation of GO sheets to form a wet layer of GO on the supporting substrate; (c) introducing a halogenating agent into the wet layer of graphene oxide and effecting a chemical reaction between the halogenating agent and GO sheets to form a wet layer of halogenated graphene, C6ZxOy, wherein Z is a halogen element selected from F, Cl, Br, I, or a combination thereof, x=0.01 to 6.0, y=0 to 5.0, and x+y≤6.0; and (d) removing the fluid medium.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. patent application Ser.No. 14/756,591, filed on Sep. 23, 2015 (now U.S. Pat. No. 9,809,459),which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of graphenematerials and, more particularly, to a new form of halogenated graphenefilm composed of originally separated multiple halogenated graphenesheets or molecules that are oriented and chemically merged andintegrated together to form a monolithic integral layer.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material). The carbonnano-tube (CNT) refers to a tubular structure grown with a single wallor multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs)have a diameter on the order of a few nanometers to a few hundrednanometers. Their longitudinal, hollow structures impart uniquemechanical, electrical and chemical properties to the material. The CNTor CNF is a one-dimensional nano carbon or 1-D nano graphite material.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of carbon atoms providedthe inter-planar van der Waals forces can be overcome. An isolated,individual graphene sheet of carbon atoms is commonly referred to assingle-layer graphene. A stack of multiple graphene planes bondedthrough van der Waals forces in the thickness direction with aninter-graphene plane spacing of approximately 0.3354 nm is commonlyreferred to as a multi-layer graphene. A multi-layer graphene platelethas up to 300 layers of graphene planes (<100 nm in thickness), but moretypically up to 30 graphene planes (<10 nm in thickness), even moretypically up to 20 graphene planes (<7 nm in thickness), and mosttypically up to 10 graphene planes (commonly referred to as few-layergraphene in scientific community). Single-layer graphene and multi-layergraphene or graphene oxide sheets are collectively called “nano grapheneplatelets” (NGPs). Graphene or graphene oxide sheets/platelets(collectively, NGPs) are a new class of carbon nano material (a 2-D nanocarbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

Isolated or separated graphene or graphene oxide sheets (NGPs) aretypically obtained by intercalating natural graphite particles with astrong acid and/or oxidizing agent to obtain a graphite intercalationcompound (GIC) or graphite oxide (GO), as illustrated in FIG. 5(A)(process flow chart) and FIG. 5(B) (schematic drawing). The presence ofchemical species or functional groups in the interstitial spaces betweengraphene planes serves to increase the inter-graphene spacing (d₀₀₂, asdetermined by X-ray diffraction), thereby significantly reducing the vander Waals forces that otherwise hold graphene planes together along thec-axis direction. The GIC or GO is most often produced by immersingnatural graphite powder (20 in FIG. 5(A) and 100 in FIG. 5(B)) in amixture of sulfuric acid, nitric acid (an oxidizing agent), and anotheroxidizing agent (e.g. potassium permanganate or sodium perchlorate). Theresulting GIC (22 or 102) is actually some type of graphite oxide (GO)particles. This GIC or GO is then repeatedly washed and rinsed in waterto remove excess acids, resulting in a graphite oxide suspension ordispersion, which contains discrete and visually discernible graphiteoxide particles dispersed in water. There are two processing routes tofollow after this rinsing step:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (24 or 104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected. A SEM image of graphite worms is presented inFIG. 6(A).

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbonmaterial (graphene sheets or platelets, NGPs). Flexible graphite (FG)foils can be used as a heat spreader material, but exhibiting a maximumin-plane thermal conductivity of typically less than 500 W/mK (moretypically <300 W/mK) and in-plane electrical conductivity no greaterthan 1,500 S/cm. These low conductivity values are a direct result ofthe many defects, wrinkled or folded graphite flakes, interruptions orgaps between graphite flakes, and non-parallel flakes (e.g. SEM image inFIG. 6(B), wherein many flakes are inclined at an angle deviating fromthe desired orientation direction by >30°). Many flakes are inclinedwith respect to one another at a very large angle (e.g. mis-orientationof 20-40 degrees). The average deviation angle is greater than 10°, moretypically >20°, and often >30°.

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than20 nm.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% byweight.

For the purpose of defining the claims of the instant application,graphene or NGPs include discrete (isolated or separated)sheets/platelets of single-layer and multi-layer pristine graphene,graphene oxide, or reduced graphene oxide (RGO). Pristine graphene hasessentially 0% oxygen. RGO typically has an oxygen content of 0.001%-5%by weight. Graphene oxide (including RGO) can have 0.001%-50% by weightof oxygen.

Isolated solid NGPs (i.e. discrete and separate sheets/platelets ofpristine graphene, GO, and RGO having a typical length/width of 100 nmto 10 μm), when packed into a macroscopic-size film, membrane, or papersheet (34 or 114 in FIG. 5(A) or FIG. 5(B)) by using, for instance, apaper-making process, typically do not exhibit a high thermalconductivity, as an example of useful physical properties. This ismainly ascribed to the notion that these sheets/platelets are typicallypoorly oriented and many types of defects are formed in thefilm/membrane/paper. Specifically, a paper-like structure or mat madefrom platelets of graphene, GO, or RGO (e.g. those paper sheets preparedby vacuum-assisted filtration process) exhibit many defects, wrinkled orfolded graphene sheets, interruptions or gaps between platelets, andnon-parallel platelets (e.g. SEM image in FIG. 7(B)), leading torelatively poor thermal conductivity, low electric conductivity, lowdielectric breakdown strength, and low structural strength. Thesepaper-like structures or aggregates of discrete NGP, GO or RGO plateletsalone (without a resin binder) also have a tendency to get flaky,emitting conductive particles into air; but the presence of a binderresin significantly reduces the conductivity of the structure.

Graphene thin films (<5 nm and most typically <2 nm) can be prepared bycatalytic chemical vapor deposition CVD of hydrocarbon gas (e.g. C₂H₄)on Ni or Cu surface. With Ni or Cu being the catalyst, carbon atomsobtained via decomposition of hydrocarbon gas molecules at 800-1,000° C.are deposited onto Ni or Cu foil surface to form a sheet of single-layeror few-layer graphene (2-5 layers in this case) that ispoly-crystalline. These ultra-thin graphene films, being opticallytransparent and electrically conducting, are intended for applicationssuch as the touch screen (to replace indium-tin oxide or ITO glass) orsemiconductor devices (to replace silicon, Si). The Ni- or Cu-catalyzedCVD process does not lend itself to the deposition of more than 5-10graphene planes (typically <5 nm and more typically <2 nm) beyond whichthe underlying Ni or Cu catalyst can no longer provide any catalyticeffect. There has been no experimental evidence to indicate that CVDgraphene film thicker than 5 nm is possible. Furthermore, the CVDprocess is known to be extremely expensive.

From semiconductor physics perspectives, on the one hand, multi-layergraphene sheets are a metallic or conductor material and single-layergraphene sheets are a semi-metal. The single-layer pristine graphenelacks an energy band gap because its valence and conduction bands toucheach other and, hence, it is labeled as a semimetal. (In contrast, Si, asemiconductor, has an energy band gap of 1.1 eV between the conductionband and valence band of its electronic configuration.) The lack of aband gap limits usage of graphene in contemporary electronic devices.The band structure of single-layer graphene can be modified to open theband gap by many strategies, e.g., halogenation, oxidation,hydrogenation or noncovalent attachment of various molecules andspecies.

On the other hand, heavily oxidized graphene or graphene oxide (GO) isconsidered an insulating material and presumably can be used as adielectric. However, the low thermal stability of GO (against heatexposure) reduces its dielectric resistivity, which is a drawback sincethermal processing steps are often used during electronic devicefabrication. Furthermore, multi-layer pristine graphene (>3 layers) andreduced graphene oxide (RGO) are essentially a conductor that cannot beused as a dielectric material or an insulating material.

Dielectric materials have attracted great attention because of theirpotential application in gate dielectrics, dynamic random access memory,artificial muscles and energy storage devices. Dielectric (ceramic)capacitors for energy storage suffer from poor processability (e.g.processing temperature usually exceeds 1,000° C.), high density, and lowbreakdown strength. Traditional high dielectric perovskite ceramics,such as barium titanate-containing composites, cannot be used atsituations where diverse shapes are required. Compared with inorganicceramics, polymers are more applicable in higher electric fields.Additionally, polymers possess the following advantages over inorganicceramics: low weight, low cost, ease in processing, and self-healing.However, low operation temperatures restrict the further development ofpolymer dielectrics. Commercial capacitors are only used in limitedapplications such as cell phones, video/audio systems, and personalcomputers. For example, biaxial oriented polypropylene polymer-basedcapacitors can only be operated at temperatures below 105° C. Thus,materials with high dielectric constants, especially those which can beused in high temperature environments, have great potential in deviceapplications.

With these drawbacks of current dielectric materials in mind, weproceeded to investigate the potential of using graphene-derivedmaterials for dielectric applications. After an in-depth and extensivestudy, we have surprisingly discovered that halogenated graphenematerials thicker than 10 nm are a good dielectric material. Halogenatedgraphene is a group of graphene derivatives, in which some carbon atomsare covalently linked with halogen atoms. The carbon atoms linked withhalogens have sp³ hybridization and other carbon atoms have sp²hybridization. This implies that halogenated graphene (also referred toas graphene halide) potentially can be an insulating material. For thispurpose, thicker graphene halide films (>10 nm, preferably >100 nm,further preferably >1 μm, and more preferably >10 μm) are desired.However, although ultra-thin films (e.g. <<10 nm) of graphene fluoridehave been produced by the catalytic CVD preparation of pristinegraphene, followed by fluorination, thicker graphene fluoride filmshaving a combination of desired physical and chemical properties havenot been available. It is known in the art that thicker dielectricmaterials (thicker than 5-10 nm) tend to have low dielectric breakdownstrength even though most of the current devices demand to have thickerdielectric components.

Thus, it is an object of the present invention to provide acost-effective process for producing thicker films of graphene-derivedmaterials that exhibit a high dielectric breakdown strength, highdielectric constants, adequate mechanical strength, good thermalstability, and good chemical stability.

SUMMARY OF THE INVENTION

The present invention provides a process for producing an integratedlayer of highly oriented halogenated graphene sheets or molecules,wherein the film has a thickness from 10 nm to 500 μm. The processcomprises: (a) preparing either a graphene oxide dispersion havinggraphene oxide sheets dispersed in a fluid medium or a graphene oxidegel having graphene oxide molecules dissolved in a fluid medium, whereinthe graphene oxide sheets or graphene oxide molecules contain an oxygenamount higher than 5% by weight; (b) dispensing and depositing a layerof graphene oxide dispersion or graphene oxide gel onto a surface of asupporting substrate under a shear stress condition, wherein thedispensing and depositing procedure includes shear-induced thinning ofgraphene oxide dispersion or gel and shear-induced orientation ofgraphene oxide sheets or molecules, to form a wet layer of grapheneoxide on the supporting substrate; (c) either (i) introducing ahalogenating agent into the wet layer of graphene oxide and effecting achemical reaction between the halogenating agent and graphene oxidesheets or molecules to form a wet layer of halogenated graphene andremoving the fluid medium from the wet layer of halogenated graphene, or(ii) removing the fluid medium from the wet layer of graphene oxide toform a dried layer of graphene oxide and introducing a halogenatingagent into the dried layer of graphene oxide and effecting a chemicalreaction between the halogenating agent and the graphene oxide sheets ormolecules, to form a dried integrated layer of halogenated graphenehaving a chemical formula of C₆Z_(x)O_(y), wherein Z is a halogenelement selected from F, Cl, Br, I, or a combination thereof, x=0.01 to6.0, y=0 to 5.0, and x+y≤6.0; and (d) removing the fluid medium from thewet layer of halogenated graphene to form the integrated layer ofhalogenated graphene having an inter-planar spacing d₀₀₂ of 0.35 nm to1.2 nm (more typically 0.40-1.0 nm) as determined by X-ray diffraction.

It may be noted that the inter-planar spacing range of 0.350-1.2 nm, asopposed to the typically 0.3359 nm of original natural graphite is dueto the presence of halogen elements or halogen-containing chemicalgroups (plus some residual O or O-containing groups in some cases) ongraphene planes that push the neighboring planes apart.

It may be noted that, in terms of timing sequence, the halogenatingagent can be introduced before, during, or after removal of the liquidmedium from the wet layer of GO.

The graphene halide sheets in the dried integrated layer of graphenehalide are substantially parallel to one another along one direction andthe average deviation angle of these sheets is less than 10 degrees. Itmay be noted that, in conventional GO or RGO sheet-based paper, graphenesheets or platelets are inclined with respect to one another at a verylarge angle (e.g. mis-orientation of 20-40 degrees). The averagedeviation angle from the desired orientation angle is greater than 10°,more typically >20°, and often >30°.

We have discovered that the electric and dielectric properties of GOfilms rapidly degrade as the aging or heat treatment temperature exceeds100° C. For instance, the electric resistivity could decrease from 10⁻⁶Ω-cm to 10⁺² Ω-cm if GO films are exposed to heat at 200° C. for a fewhours; this is an 8 orders of magnitude drop in resistivity and GObecomes a totally useless dielectric material. In contrast, thepresently invented integrated films of highly oriented halogenatedgraphene, C₆Z_(x)O_(y), can be thermal stable up to 1,000-2,500° C.,depending on the chemical composition. A higher x value or the x/(y+x)ratio leads to a higher maximum useful temperature. When x=1, thethermal stability temperature can be as high as 2,000-2,500° C.

In certain embodiments, timing-wise, step (b) can occur during or afterstep (b). Thus, the invention also provides a process which includes (a)preparing either a graphene oxide dispersion having graphene oxidesheets dispersed in a fluid medium or a graphene oxide gel havinggraphene oxide molecules dissolved in a fluid medium, wherein saidgraphene oxide sheets or graphene oxide molecules contain an oxygenamount higher than 5% by weight; (b) introducing a halogenating agentinto said graphene oxide dispersion or gel and effecting a chemicalreaction between said halogenating agent and said graphene oxide sheetsor molecules to form a dispersion of halogenated graphene sheets or agel of halogenated graphene molecules, wherein said halogenated graphenesheets have a chemical formula of C₆Z_(x)O_(y), wherein Z is a halogenelement selected from F, Cl, Br, I, or a combination thereof, x=0.01 to6.0, y=0 to 5.0, and x+y≤6.0; (c) dispensing and depositing a layer ofsaid halogenated graphene dispersion or gel onto a surface of asupporting substrate under a shear stress condition, wherein saiddispensing and depositing procedure includes shear-induced thinning ofsaid halogenated graphene dispersion or gel and shear-inducedorientation of halogenated graphene sheets or molecules, to form a wetlayer of halogenated graphene on said supporting substrate; and (d)removing said fluid medium from the wet layer of halogenated graphene toform said integrated layer of halogenated graphene having aninter-planar spacing d₀₀₂ of 0.35 nm to 1.2 nm as determined by X-raydiffraction. Further, graphene halide sheets in the dried integratedlayer of graphene halide are substantially parallel to one another alongone direction and the average deviation angle of these sheets is lessthan 10 degrees.

In various embodiments, the starting graphene oxide sheets containsingle-layer graphene oxide or few-layer graphene oxide sheets eachhaving 2-10 oxidized graphene planes. The fluid medium can be water, analcohol, a mixture of water and alcohol, or an organic solvent.

The fluorinating agent preferably contains a chemical species in aliquid, gaseous, or plasma state containing a halogen element selectedfrom F, Cl, Br, I, or a combination thereof. In certain embodiments, thefluorinating agent contains a chemical species in a liquid, gaseous, orplasma state containing a halogen element selected from F, Cl, Br, I, ora combination thereof. In particularly preferred embodiments, thefluorinating agent is selected from hydrofluoric acid,hexafluorophosphoric acid or HPF₆, XeF₂, F₂ gas, F₂/Ar plasma, CF₄plasma, SF₆ plasma, HCl, HPCl₆, XeCl₂, Cl₂ gas, Cl₂/Ar plasma, CCl₄plasma, SCl₆ plasma, HBr, XeBr₂, Br₂ gas, Br₂/Ar plasma, CBr₄ plasma,SBr₆ plasma, HI, XeI₂, I₂, I₂/Ar plasma, Cl₄ plasma, SI₆ plasma, or acombination thereof.

The step of dispensing and depositing can include a printing, spraying,coating and/or casting procedure, which is in combination with a shearstress procedure. The coating process can include a slot die coating orcomma coating procedure. More preferably, the dispensing and depositingstep includes a reverse roll transfer coating procedure.

In some preferred versions of reverse roll transfer coating process, thestep of dispensing and depositing includes dispensing the layer ofgraphene oxide dispersion or graphene oxide gel onto a surface of anapplication roller rotating in a first direction at a first linevelocity to form an applicator layer of graphene oxide, wherein theapplication roller transfers the applicator layer of graphene oxide to asurface of a supporting film driven in a second direction opposite tothe first direction at a second line velocity, to form the wet layer ofgraphene oxide on the supporting film. The supporting film may be drivenby a counter-rotating supporting roller disposed at a working distancefrom the application roller and rotating in the second directionopposite to the first direction.

The velocity ratio, defined as (said second line velocity)/(said firstline velocity), is preferably from 1/5 to 5/1, more preferably isgreater than 1/1 and less than 5/1. If the external surface of theapplication roller moves at the same speed as the linear movement speedof the supporting film, then the velocity ratio is 1/1 or unity. If, asan example, the external surface of the application roller moves at aspeed three times as fast as the linear movement speed of the supportingfilm, then the velocity ratio is 3/1. In certain embodiments, thevelocity ratio is greater than 1/1 and less than 5/1. Preferably, thevelocity ratio is greater than 1/1 and up to 3/1.

In certain embodiments, the step of dispensing graphene oxide dispersionor graphene oxide gel onto the surface of the application rollerincludes using a metering roller and/or a doctor's blade to provide adesired thickness of the applicator layer of graphene oxide on theapplication roller surface. The process can include operating 2, 3, or 4rollers.

In a preferred embodiment, the supporting film is driven by acounter-rotating supporting roller disposed at a working distance fromthe application roller and rotating in the second direction opposite tothe first direction. The speed at the external surface of thissupporting roller dictates the second line velocity (of the supportingfilm). Preferably, the supporting film is fed from a feeder roller andthe dried layer of graphene halide supported by the supporting film iswound on a winding roller and the process is conducted in a roll-to-rollmanner.

Preferably, the invented process further comprises a step of aging thewet layer of graphene oxide after step (b), aging the wet layer ofhalogenated graphene after step (c), or aging the integrated layer ofhalogenated graphene after step (d), in an aging room at an agingtemperature from 25° C. to 100° C. and humidity level from 20% to 99%for an aging time of 1 hour to 7 days.

The process may further comprise a compression step, during or aftersaid step (d), to reduce a thickness of said integrated layer.

The process may further comprise a step (e) of heat treating theintegrated layer of oriented halogenated graphene at a first heattreatment temperature higher than 100° C. but no greater than 3,200° C.for a desired length of time to produce a graphitic film having aninter-planar spacing d₀₀₂ less than 0.4 nm and a combined oxygen/halogencontent less than 1% by weight.

The process may further comprise a compression step, during or after theheat treatment step, to reduce a thickness of the graphitic film.

In the invented process, the graphene oxide sheets in the graphene oxidedispersion preferably occupy a weight fraction of 0.1% to 25% based onthe total weight of graphene oxide sheets and liquid medium combined.More preferably, the graphene oxide sheets in the graphene oxidedispersion occupy a weight fraction of 0.5% to 15%. In some embodiments,graphene oxide sheets occupy a weight proportion from 3% to 15% based onthe total weight of graphene oxide sheets and liquid medium combined. Incertain embodiments, the graphene oxide dispersion or graphene oxide gelhas greater than 3% by weight of graphene oxide dispersed in the fluidmedium to form a liquid crystal phase.

The graphene oxide dispersion or graphene oxide gel may be prepared byimmersing a graphitic material in a powder or fibrous form in anoxidizing liquid in a reaction vessel at a reaction temperature for alength of time sufficient to obtain said graphene oxide dispersion orsaid graphene oxide gel wherein said graphitic material is selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.

The graphene oxide dispersion or graphene oxide gel may be obtained froma graphitic material having a maximum original graphite grain size andthe resulting halogenated graphene film is a poly-crystal graphenestructure having a grain size larger than this maximum original grainsize. This larger grain size is due to the notion that heat-treating ofGO sheets, GO molecules, halogenated molecules, or halogenate sheetsinduces chemical linking, merging, or chemical bonding of grapheneoxide/halide sheets or graphene oxide/halide molecules in anedge-to-edge manner. It may be noted that such an edge-to-edge linkingsignificantly increases the length or width of graphene sheets ormolecules. For instance, a graphene halide sheet 300 nm in length, ifmerged with a graphene halide sheet 400 nm in length could result in asheet of approximately 700 nm in length. Such an edge-to-edge merging ofmultiple graphene halide sheets enables production of graphene filmshaving huge grain sizes that could not be obtained otherwise.

In an embodiment, the graphene oxide dispersion or graphene oxide gel isobtained from a graphitic material having multiple graphite crystallitesexhibiting no preferred crystalline orientation as determined by anX-ray diffraction or electron diffraction method and the resultinghalogenated graphene film is a single crystal or a poly-crystal graphenestructure having a preferred crystalline orientation as determined bysaid X-ray diffraction or electron diffraction method.

All the coating procedures capable of inducing shear stresses to the GOsheets or halogenated GO sheets may be implemented in the presentlyinvented process; e.g. slot-die coating, comma coating, and reverse rolltransfer coating. The reverse roll procedure is particularly effectivein enabling the GO sheets or GO molecules to align themselves along aparticular direction (e.g. X-direction or length-direction) or twoparticular directions (e.g. X- and Y-directions or length and widthdirections) to produce preferred orientations. Further surprisingly,these preferred orientations are preserved and often further enhancedduring the subsequent heat treatment of the GO layers. Mostsurprisingly, such preferred orientations are essential to the eventualattainment of exceptionally high dielectric breakdown strength,dielectric constants, elastic modulus, and tensile strength of theresulting halogenated graphene film (even for thick films; e.g. from 10nm to even >500 μm) along a desired direction. During the coating orcasting processes, other than the presently invented reverse rollprocedure-based process, the thickness of the coated or cast films(layers) cannot be too high (e.g. greater than 50 μm), otherwise a highdegree of GO or halogenated sheet orientation cannot be achieved. Ingeneral, in the conventional casting or comma coating processes, thecoated or cast films (wet layers) must be sufficiently thin so that whenthey become dried, they form a dried layer of graphene oxide having athickness no greater than 50 μm, more typically no greater than 20 μm,and most typically no greater than 10 μm. Through extensive and in-depthexperimental studies we have come to unexpectedly realize that thereverse roll procedure is so effective in achieving and maintaining ahigh degree of preferred orientation even for very thick films.

The integrated layer of oriented halogenated graphene herein producedtypically has a dielectric constant greater than 4.0 (more typicallygreater than 5.0, often greater than 10, or even greater than 15), anelectrical resistivity typically from 10⁸ Ω-cm to 10¹⁵ Ω-cm, and/or adielectric breakdown strength greater than 5 MV/cm (more typicallygreater than 10 MV/cm, some greater than 12 MV/cm, and others evengreater than 15 MV/cm) when measured at a layer thickness of 100 nm.

The present invention also provides a microelectronic device containingthe integrated layer of halogenated graphene as a dielectric component.

This new class of materials (i.e., highly oriented GO-derived graphenehalide films, GOGH) have the following characteristics that distinguishthemselves from paper/film/membrane layers of discretegraphene/GO/RGO/GH sheets/platelets:

-   -   (1) This GOGH film is an integrated graphene halide (GH) entity        that is a poly-crystalline structure composed of well-aligned,        interconnected multiple grains with exceptionally large grain        sizes. The HOGH has all the graphene planes in all the grains        being essentially oriented parallel to one another (i.e., the        crystallographic c-axis of all grains essentially pointing in an        identical direction).    -   (2) With a reverse roll procedure, an exceptionally high degree        of orientation of GH platelets can be achieved even with thick        films (>10 nm), not just thin films. Given the same thickness,        the reverse roll procedure enables a higher degree of        orientation and higher degree of crystal perfection.    -   (3) The GOGH is an integrated graphene entity that is not a        simple aggregate or stack of multiple discrete platelets of        graphene/GO/RGO/GH (graphene halide), and does not contain any        discernible or discrete flake/platelet derived from the original        GO sheets. These originally discrete flakes or platelets have        been chemically bonded or linked together to form larger grains        (grain size being larger than the original platelet/flake size).    -   (4) This GOGH is not made by using a binder or adhesive to glue        discrete flakes or platelets together. Instead, under select        aging or heat treatment conditions, well-aligned GO/GH sheets or        GO/GH molecules are capable of chemically merging with one        another mainly in an edge-to-edge manner to form giant 2-D        graphene grains, but possibly also with adjacent GO/GH sheets        below or above to form 3-D networks of graphene chains. Through        joining or forming of covalent bonds with one another, the GO/GH        sheets are adhered into an integrated graphene entity, without        using any externally added linker or binder molecules or        polymers.    -   (5) This GOGH, a poly-crystal with essentially all graphene        planes having an identical crystallographic c-axis, is derived        from GO, which is in turn obtained from moderate or heavy        oxidation of natural graphite or artificial graphite particles        each originally having multiple graphite crystallites that are        randomly oriented. Prior to being chemically oxidized to become        GO dispersion (moderate-to-heavy oxidation of graphite) or GO        gel (heavy oxidation for a sufficiently long oxidation time to        achieve fully separated GO molecules dissolved in water or other        polar liquid), these starting or original graphite crystallites        have an initial length (L_(a) in the crystallographic a-axis        direction), initial width (L_(b) in the b-axis direction), and        thickness (L_(c) in the c-axis direction). The resulting GOGH        typically has a length or width significantly greater than the        L_(a) and L_(b) of the original graphite crystallites.    -   (6) This process for producing a monolithic, integrated layer of        highly oriented GH sheets can be conducted on a continuous        roll-to-roll basis and, hence, is a scalable, cost-effective        process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a reverse roll-based GO/GH layer transfer apparatusfor producing highly oriented GO/GH films.

FIG. 2 Schematic of another reverse roll-based GO/GH layer transferapparatus for producing highly oriented GO/GH films.

FIG. 3 Schematic of yet another reverse roll-based GO/GH layer transferapparatus for producing highly oriented GO/GH films.

FIG. 4 Schematic of still another reverse roll-based GO/GH layertransfer apparatus for producing highly oriented GO/GH films.

FIG. 5(A) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andflexible graphite composites) and pyrolytic graphite (bottom portion),along with a process for producing graphene oxide gel or GO dispersion.

FIG. 5(B) Schematic drawing illustrating the processes for producingconventional paper, mat, film, and membrane of simply aggregatedgraphite or NGP flakes/platelets. All processes begin with intercalationand/or oxidation treatment of graphitic materials (e.g. natural graphiteparticles).

FIG. 6(A) A SEM image of a graphite worm sample after thermalexfoliation of graphite intercalation compounds (GICs) or graphite oxidepowders.

FIG. 6(B) An SEM image of a cross-section of a flexible graphite foil,showing many graphite flakes with orientations not parallel to theflexible graphite foil surface and also showing many defects, kinked orfolded flakes.

FIG. 7(A) A SEM image of a GO-derived film, wherein multiple grapheneplanes (having an initial length/width of 30 nm-300 nm in originalgraphite particles) have been oxidized, exfoliated, re-oriented, andseamlessly merged into continuous-length graphene sheets or layers thatcan run for tens of centimeters wide or long (only a 50 μm width of a10-cm wide graphitic film being shown in this SEM image).

FIG. 7(B) A SEM image of a cross-section of a conventional graphenepaper/film prepared from discrete graphene sheets/platelets using apaper-making process (e.g. vacuum-assisted filtration). The image showsmany discrete graphene sheets being folded or interrupted (notintegrated), with orientations not parallel to the film/paper surfaceand having many defects or imperfections.

FIG. 7(C) Schematic drawing and an attendant SEM image to illustrate theformation process of a HOGF that is composed of multiple graphene planesthat are parallel to one another and are chemically bonded in thethickness-direction or crystallographic c-axis direction.

FIG. 7(D) One plausible chemical linking mechanism (only 2 GO moleculesare shown as an example; a large number of GO molecules can bechemically linked together to form a graphene layer).

FIG. 8 Dielectric breakdown strength of GO film, GO-derived graphenefluoride film (made by the reverse roll transfer coating process), andpolyimide film plotted as a function of the film thickness.

FIG. 9 Dielectric breakdown strength of GO-derived fluorinated graphenefilms and chlorinated graphene films (both prepared by a reverse rolltransfer procedure and a casting procedure) plotted as a function of thedegree of fluorination (atomic ratio, F/(F+O)) or the degree ofchlorination (atomic ratio, Cl/(Cl+O)).

FIG. 10 Dielectric breakdown strength of GO-derived fluorinated graphenefilms (prepared by a reverse roll transfer procedure) and GO-derivedfluorinated graphene paper prepared by the conventional paper-makingprocedure (vacuum-assisted filtration) plotted as a function of thedegree of fluorination in terms of the atomic ratio, F/(F+O).

FIG. 11 Dielectric constants of GO-derived fluorinated graphene filmsand brominated graphene films plotted as a function of the degree offluorination (atomic ratio, F/(F+O)) or the degree of bromination(atomic ratio, Br/(Br+O)).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Provided is a process for producing a monolithic (single-material orsingle-phase) integrated layer of highly oriented halogenated graphenesheets or molecules, wherein the film has a thickness from 10 nm to 500μm. The halogenated graphene has a chemical formula of C₆Z_(x)O_(y),wherein Z is a halogen element selected from F, Cl, Br, I, or acombination thereof, x=from 0.01 to 6.0, y=from 0 to 5.0, and x+y≤6.0.The preparation of this integrated layer begins with graphene oxide (GO)in a suspension (dispersion) or gel form. Specifically, the processbegins with (a) preparing either a graphene oxide (GO) dispersion havinggraphene oxide sheets dispersed in a fluid medium or a graphene oxidegel having graphene oxide molecules dissolved in a fluid medium, whereinthe graphene oxide sheets or graphene oxide molecules contain an oxygenamount higher than 5% by weight (typically from 5% to 46%, butpreferably from 10% to 46% and more preferably from 20% to 46%).

This is followed by step (b), which entails dispensing and depositing alayer of graphene oxide dispersion or graphene oxide gel onto a surfaceof a supporting substrate under a shear stress condition, wherein thedispensing and depositing procedure includes shear-induced thinning ofgraphene oxide dispersion or gel and shear-induced orientation ofgraphene oxide sheets or molecules, to form a wet layer of grapheneoxide on the supporting substrate. This step includes spraying,printing, extruding, casting, and/or coating of a wet layer of GO onto asolid substrate surface (e.g. a PET film, Al foil, glass surface, etc.)that contains or is followed by a shearing procedure. The presence of ashear stress is essential to aligning GO sheets or molecules along adesired direction.

The third step, step (c), is then conducted to chemically replace O oroxygen-containing functional group with a halogen element orhalogen-containing group. Halogen herein refers to F, Cl, Br, I, or acombination thereof. Thus, this step (c) entails introducing ahalogenating agent into the wet layer of graphene oxide and effecting achemical reaction between the halogenating agent and the graphene oxidesheets or molecules to form a wet layer of halogenated graphene having achemical formula of C₆Z_(x)O_(y), wherein Z is a halogen elementselected from F, Cl, Br, I, or a combination thereof, x=0.01 to 6.0, y=0to 5.0, and x+y≤6.0. The halogenating agent may contain a chemicalspecies in a liquid, gaseous, or plasma state containing a halogenelement selected from F, Cl, Br, I, or a combination thereof.Halogenated graphene is a group of graphene derivatives, in which somecarbon atoms are covalently linked with halogen atoms. The carbon atomslinked with halogens have sp³ hybridization and other carbon atoms havesp² hybridization. The physical and chemical properties of halogenatedgraphene (also referred to as graphene halide) are strongly dependent onthe degree of halogenation.

Step (c) is then followed by step (d) of removing the fluid medium fromthe wet layer of halogenated graphene to form the integrated layer ofhalogenated graphene having an inter-planar spacing d₀₀₂ of 0.35 nm to1.2 nm as determined by X-ray diffraction. The removal of liquid fluidmay be conducted before, during, or after the halogenating reaction.

For accomplishing the halogenating reaction, for instance, hydrofluoricacid or hexafluorophosphoric acid (HPF₆) liquid may be injected into theGO suspension or GO gel stream before, during, or after thedispensing/depositing stage. Alternatively, F₂ gas, Cl₂ gas, Br₂ gas,and/or I₂ gas (vapor) may be introduced to a chamber where the wet GOlayer is contained, enabling the halogen gas molecules to permeate intothe wet GO layer and reacting with GO therein and thereon. Furtheralternatively, one may choose to remove the liquid medium from the wetlayer of GO to form a dried layer of GO prior to introducing thehalogenating agent to react with GO. The dried layer of GO maypreferably be treated with halogen-containing gas or plasma.

Specifically, the fluorinating agent may be selected from hydrofluoricacid, hexafluorophosphoric acid or HPF₆, XeF₂, F₂ gas, F₂/Ar plasma, CF₄plasma, SF₆ plasma, HCl, HPCl₆, XeCl₂, Cl₂ gas, Cl₂/Ar plasma, CCl₄plasma, SCl₆ plasma, HBr, XeBr₂, Br₂ gas, Br₂/Ar plasma, CBr₄ plasma,SBr₆ plasma, HI, XeI₂, I₂, I₂/Ar plasma, CI₄ plasma, SI₆ plasma, or acombination thereof.

A highly preferred dispensing and depositing procedure is the reverseroll transfer coating, which intrinsically induces high shear stressesto the suspension or gel coated on the rollers. As schematicallyillustrated in FIG. 1, as a preferred embodiment, the process ofproducing the monolithic, integrated layer of highly oriented graphenehalide (HOGH) begins with preparation of a graphene oxide dispersion (GOdispersion) or graphene oxide gel (GO gel) that is delivered to a trough208. The rotational motion of an application roller 204 in a firstdirection enables the delivery of a continuous layer 210 of GOdispersion or gel onto the exterior surface of the application roller204. An optional doctor's blade 212 is used to regulate the thickness(amount) of an applicator layer 214 of graphene oxide (GO). Thisapplicator layer is continuously delivered to the surface of asupporting film 216 moving in a second direction (e.g. driven by acounter-rotating roller 206, rotating in a direction opposite to thefirst direction) to form a wet layer 218 of graphene oxide. This wetlayer of GO is then subjected to a liquid removal treatment (e.g. undera heating environment and/or being vacuum-pumped).

In summary, the process begins with preparation of either a grapheneoxide dispersion having graphene oxide sheets dispersed in a fluidmedium or a graphene oxide gel having graphene oxide molecules dissolvedin a fluid medium, wherein the graphene oxide sheets or graphene oxidemolecules contain an oxygen content higher than 5% by weight. Thegraphene oxide dispersion or graphene oxide gel is then dispensed anddelivered onto a surface of an application roller rotating in a firstdirection at a first line velocity (the line speed at the externalsurface of the application roller) to form an applicator layer ofgraphene oxide and transferring this applicator layer of graphene oxideto a surface of a supporting film driven in a second direction oppositeto the first direction at a second line velocity, forming a wet layer ofgraphene oxide on the supporting film.

In a preferred embodiment, the supporting film is driven by acounter-rotating supporting roller (e.g. 206 in FIG. 1) disposed at aworking distance from the application roller and rotating in the seconddirection opposite to the first direction. The speed at the externalsurface of this supporting roller dictates the second line velocity (ofthe supporting film). Preferably, the supporting film is fed from afeeder roller and the dried layer of graphene oxide supported by thesupporting film is wound on a winding roller and the process isconducted in a roll-to-roll manner.

This process is further illustrated in FIG. 2, FIG. 3, and FIG. 4. In apreferred embodiment, as illustrated in FIG. 2, the GO dispersion/geltrough 228 is naturally formed between an application roller 224 and ametering roller 222 (also referred to as a doctor's roller). Therelative motion or rotation of the application roller 224, relative tothe metering roller 222, at a desired speed generates an applicatorlayer 230 of GO on the exterior surface of the application roller 224.This applicator layer of GO is then transferred to form a wet layer 232of GO on the surface of a supporting film 234 (driven by a supportingroller 226 counter-rotating in a direction opposite to the rotationaldirection of the applicator roller 224). The wet layer may then besubjected to halogenating and drying treatments.

In another preferred embodiment, as illustrated in FIG. 3, the GOdispersion/gel trough 244 is naturally formed between an applicationroller 238 and a metering roller 236. The relative motion or rotation ofthe application roller 238, relative to the metering roller 236, at adesired speed generates an applicator layer 248 of GO on the exteriorsurface of the application roller 238. A doctor's blade 242 may be usedto scratch off any GO gel/dispersion carried on the exterior surface ofthe metering roller 236. This applicator layer 248 of GO is thentransferred to form a wet layer 250 of GO on the surface of a supportingfilm 246 (driven by a supporting roller 240 counter-rotating in adirection opposite to the rotational direction of the applicator roller238). The wet layer may then be subjected to halogenating and dryingtreatments.

In yet another preferred embodiment, as illustrated in FIG. 4, the GOdispersion/gel trough 256 is naturally formed between an applicationroller 254 and a metering roller 252. The relative motion or rotation ofthe application roller 254, relative to the metering roller 252, at adesired speed generates an applicator layer 260 of GO on the exteriorsurface of the application roller 254. This applicator layer 260 of GOis then transferred to form a wet layer 262 of GO on the surface of asupporting film 258, driven to move in a direction opposite to thetangential rotational direction of the applicator roller 254. Thissupporting film 258 may be fed from a feeder roller (not shown) andtaken up (wound) on a winding roller (not shown), which may also be adriving roller. There would be at least 4 rollers in this example. Thewet layer may then be subjected to halogenating and drying treatments.For liquid medium removal, there can be a heating zone after the wetlayer of GO is formed to at least partially remove the liquid medium(e.g. water) from the wet layer to form a dried layer of GO.

In some embodiments, the step of dispensing the graphene oxidedispersion or graphene oxide gel onto the surface of the applicationroller includes using a metering roller and/or a doctor's blade toprovide a desired thickness of the applicator layer of graphene oxide onthe application roller surface. In general, the process includesoperating 2, 3, or 4 rollers. Preferably, the process includes a reverseroll coating procedure.

It may be noted that the velocity ratio, defined as (the second linevelocity)/(first line velocity), is from 1/5 to 5/1. If the externalsurface of the application roller moves at the same speed as the linearmovement speed of the supporting film, then the velocity ratio is 1/1 orunity. If, as an example, the external surface of the application rollermoves at a speed three times as fast as the linear movement speed of thesupporting film, then the velocity ratio is 3/1. As a consequence, thetransferred wet layer of GO would be approximately 3-fold in thicknessas compared to the applicator layer of GO. Quite unexpectedly, thisenables the production of much thicker layer yet still maintaining ahigh degree of GO orientation in the wet layer and the dried layer. Thisis a highly significant and desirable outcome since a high degree of GOsheet orientation could not be achieved with thick films (e.g. >50 μm inthickness) by using casting or other coating techniques, such as commacoating and slot-die coating. In certain embodiments, the velocity ratiois greater than 1/1 and less than 5/1. Preferably, the velocity ratio isgreater than 1/1 and equal to or less than 3/1. The slot-die coating orcomma coating is also capable of applying a shear stress to induce therequired orientation of GO or GH sheets or molecules.

Preferably, the process further comprises a step of aging the wet ordried layer of graphene oxide in an aging room at an aging temperaturefrom 25° C. to 200° C. (preferably from 25° C. to 100° C. and morepreferably from 25° C. to 55° C.) and humidity level from 20% to 99% foran aging time of 1 hour to 7 days to form an aged layer of grapheneoxide. We have surprisingly observed that this aging procedure enablessome chemical linking or merging of GO sheets or molecules in anedge-to-edge manner, as manifested by the observation by microscopy thatthe average length/width of the GO sheets is significantly increased (bya factor of 2-3) after aging. This would make it possible to maintainthe sheet orientation and accelerate subsequent edge-to-edge linking tohuge grains or crystal domains.

In some embodiments, the process further comprises a step of heattreating the dried or dried and aged layer of graphene oxide at a firstheat treatment temperature higher than 100° C. but no greater than3,200° C. (preferably no greater than 2,500° C.) for a desired length oftime to produce a film having an inter-planar spacing d₀₀₂ less than 0.4nm and an oxygen and/or halogen content less than 5% by weight. Theprocess can further comprise a compression step, during or after thisheat-treating step, to reduce the thickness of the graphene film.

In the invented process, the graphene oxide sheets in the graphene oxidedispersion preferably occupy a weight fraction of 0.1% to 25% based onthe total weight of graphene oxide sheets and liquid medium combined.More preferably, the graphene oxide sheets in the graphene oxidedispersion occupy a weight fraction of 0.5% to 15%. In some embodiments,graphene oxide sheets occupy a weight proportion from 3% to 15% based onthe total weight of graphene oxide sheets and liquid medium combined. Incertain embodiments, the graphene oxide dispersion or graphene oxide gelhas greater than 3% by weight of graphene oxide dispersed in the fluidmedium to form a liquid crystal phase.

The monolithic integrated halogenated graphene film contains chemicallybonded and merged graphene planes. These planar aromatic molecules orgraphene planes (hexagonal structured carbon atoms having a desiredamount of oxygen- and/or halogen-containing group) are parallel to oneanother. The lateral dimensions (length or width) of these planes arehuge, typically several times or even orders of magnitude larger thanthe maximum crystallite dimension (or maximum constituent graphene planedimension) of the starting graphite particles. The presently inventedhalogenated graphene film is a “giant graphene crystal” or “giant planargraphene particle” having all constituent graphene planes beingessentially parallel to one another. This is a unique and new class ofmaterial that has not been previously discovered, developed, orsuggested to possibly exist.

The dried graphene halide (GH) layer has a high birefringencecoefficient between an in-plane direction and the normal-to-planedirection. The oriented graphene oxide and/or halide layer is itself avery unique and novel class of material that surprisingly has greatcohesion power (self-bonding, self-polymerizing, and self-crosslinkingcapability). These characteristics have not been taught or hinted in theprior art. The GO is obtained by immersing powders or filaments of astarting graphitic material in an oxidizing liquid medium (e.g. amixture of sulfuric acid, nitric acid, and potassium permanganate) in areaction vessel. The starting graphitic material may be selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.

When the starting graphite powders or filaments are mixed in theoxidizing liquid medium, the resulting slurry is a heterogeneoussuspension and appears dark and opaque. When the oxidation of graphiteproceeds at a reaction temperature for a sufficient length of time, thereacting mass can eventually become a suspension that appears slightlygreen and yellowish, but remain opaque. If the degree of oxidation issufficiently high (e.g. having an oxygen content between 20% and 50% byweight, preferably between 30% and 50%) and all the original grapheneplanes are fully oxidized, exfoliated and separated to the extent thateach oxidized graphene plane (now a graphene oxide sheet or molecule) issurrounded by the molecules of the liquid medium, one obtains a GO gel.The GO gel is optically translucent and is essentially a homogeneoussolution, as opposed to a heterogeneous suspension.

This GO suspension or GO gel typically contains some excess amount ofacids and can be advantageously subjected to some acid dilutiontreatment to increase the pH value (preferably >4.0). The GO suspension(dispersion) preferably contain at least 1% by weight of GO sheetsdispersed in a liquid medium, more preferably at least 3% by weight, andmost preferably at least 5% by weight. It is advantageous to have anamount of GO sheets sufficient for forming a liquid crystalline phase.We have surprisingly observed that GO sheets in a liquid crystal statehave the highest tendency to get readily oriented under the influence ofa shear stress created by a commonly used casting or coating process.

The graphene oxide suspension may be prepared by immersing a graphiticmaterial (in a powder or fibrous form) in an oxidizing liquid to form areacting slurry in a reaction vessel at a reaction temperature for alength of time sufficient to obtain GO sheets dispersed in a residualliquid. Typically, this residual liquid is a mixture of acid (e.g.sulfuric acid) and oxidizer (e.g. potassium permanganate or hydrogenperoxide). This residual liquid is then washed and replaced with waterand/or alcohol to produce a GO dispersion wherein discrete GO sheets(single-layer or multi-layer GO) are dispersed in the fluid. Thedispersion is a heterogeneous suspension of discrete GO sheets suspendedin a liquid medium and it looks optically opaque and dark (relativelylow degree of oxidation) or slightly green and yellowish (if the degreeof oxidation is high).

Now, if the GO sheets contain a sufficient amount of oxygen-containingfunctional groups and the resulting dispersion (suspension or slurry) ismechanically sheared or ultrasonicated to produce individual GO sheetsor molecules that are dissolved (not just dispersed) in water and/oralcohol or other polar solvent, we can reach a material state called “GOgel” in which all individual GO molecules are surrounded by themolecules of the liquid medium. The GO gel looks like a homogeneoussolution which is translucent and no discernible discrete GO or graphenesheets can be visibly identified. Useful starting graphitic materialsinclude natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. As the oxidizing reaction proceeds to a criticalextent and individual GO sheets are fully separated (now with grapheneplane and edges being heavily decorated with oxygen-containing groups),an optically transparent or translucent solution is formed, which is theGO gel.

Preferably, the GO sheets in such a GO dispersion or the GO molecules insuch a GO gel are in the amount of 1%-15% by weight, but can be higheror lower. More preferably, the GO sheets are 2%-10% by weight in thesuspension. Most preferably, the amount of GO sheets is sufficient toform a liquid crystal phase in the dispersing liquid. The GO sheets havean oxygen content typically in the range from 5% to 50% by weight, moretypically from 10% to 50%, and most typically from 20% to 46% by weight.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 5(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 5(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 5(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 5(A) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils andthe resin-impregnated flexible graphite composite. The processestypically begin with intercalating graphite particles 20 (e.g., naturalgraphite or synthetic graphite) with an intercalant (typically a strongacid or acid mixture) to obtain a graphite intercalation compound 22(GIC). After rinsing in water to remove excess acid, the GIC becomes“expandable graphite.” The GIC or expandable graphite is then exposed toa high temperature environment (e.g., in a tube furnace preset at atemperature in the range of 800-1,050° C.) for a short duration of time(typically from 15 seconds to 2 minutes). This thermal treatment allowsthe graphite to expand in its c-axis direction by a factor of 30 toseveral hundreds to obtain a worm-like vermicular structure 24 (graphiteworm), which contains exfoliated, but un-separated graphite flakes withlarge pores interposed between these interconnected flakes. An exampleof graphite worms is presented in FIG. 6(A).

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 5(A) or 106 inFIG. 5(B)), which are typically 100-300 μm thick. An SEM image of across-section of a flexible graphite foil is presented in FIG. 6(B),which shows many graphite flakes with orientations not parallel to theflexible graphite foil surface and there are many defects andimperfections.

Largely due to these mis-orientations of graphite flakes and thepresence of defects, commercially available flexible graphite foilsnormally have an in-plane electrical conductivity of 1,000-3,000 S/cm,through-plane (thickness-direction or Z-direction) electricalconductivity of 15-30 S/cm, in-plane thermal conductivity of 140-300W/mK, and through-plane thermal conductivity of approximately 10-30W/mK. These defects and mis-orientations are also responsible for thelow mechanical strength (e.g. defects are potential stress concentrationsites where cracks are preferentially initiated). These properties areinadequate for many thermal management applications and the presentinvention is made to address these issues.

In another prior art process, the exfoliated graphite worm 24 may beimpregnated with a resin and then compressed and cured to form aflexible graphite composite 28, which is normally of low strength aswell. In addition, upon resin impregnation, the electrical and thermalconductivity of the graphite worms could be reduced by two orders ofmagnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 5(B)). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 5(B) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm in the present application. When the platelet isapproximately circular in shape, the length and width are referred to asdiameter. In the presently defined NGPs, both the length and width canbe smaller than 1 μm, but can be larger than 200 μm.

A mass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide, 33 in FIG.5(A)) may be made into a graphene film/paper (34 in FIG. 5(A) or 114 inFIG. 5(B)) using a film- or paper-making process. FIG. 7(B) shows a SEMimage of a cross-section of a graphene paper/film prepared from discretegraphene sheets using a paper-making process. The image shows thepresence of many discrete graphene sheets being folded or interrupted(not integrated), most of platelet orientations being not parallel tothe film/paper surface, the existence of many defects or imperfections.NGP aggregates, even when being closely packed, exhibit a thermalconductivity higher than 1,000 W/mK only when the film or paper is castand strongly pressed into a sheet having a thickness lower than 10 μm. Aheat spreader in many electronic devices is normally required to bethicker than 10 μm but thinner than 35 μm).

Another graphene-related product is the graphene oxide gel 21 (FIG.5(A)). This GO gel is obtained by immersing a graphitic material 20 in apowder or fibrous form in a strong oxidizing liquid in a reaction vesselto form a suspension or slurry, which initially is optically opaque anddark. This optical opacity reflects the fact that, at the outset of theoxidizing reaction, the discrete graphite flakes and, at a later stage,the discrete graphene oxide flakes scatter and/or absorb visiblewavelengths, resulting in an opaque and generally dark fluid mass. Ifthe reaction between graphite powder and the oxidizing agent is allowedto proceed at a sufficiently high reaction temperature for a sufficientlength of time and all the resulting GO sheets are fully separated, thisopaque suspension is transformed into a brown-colored and typicallytranslucent or transparent solution, which is now a homogeneous fluidcalled “graphene oxide gel” (21 in FIG. 5(A)) that contains nodiscernible discrete graphite flakes or graphite oxide platelets. Ifdispensed and deposited using the presently invented reverse rollcoating, the GO gel undergoes molecular orientation to form a layer ofhighly oriented GO 35, which can be heat-treated to become a graphiticfilm 37.

Again, typically, this graphene oxide gel is optically transparent ortranslucent and visually homogeneous with no discernible discreteflakes/platelets of graphite, graphene, or graphene oxide dispersedtherein. In the GO gel, the GO molecules are uniformly “dissolved” in anacidic liquid medium. In contrast, suspension of discrete graphenesheets or graphene oxide sheets in a fluid (e.g. water, organic acid orsolvent) look dark, black or heavy brown in color with individualgraphene or graphene oxide sheets discernible or recognizable even withnaked eyes or using a low-magnification light microscope (100×-1,000×).

Even though graphene oxide suspension or GO gel is obtained from agraphitic material (e.g. powder of natural graphite) having multiplegraphite crystallites exhibiting no preferred crystalline orientation,as determined by an X-ray diffraction or electron diffraction method,the resulting graphitic film exhibits a very high degree of preferredcrystalline orientation as determined by the same X-ray diffraction orelectron diffraction method. This is yet another piece of evidence toindicate that the constituent graphene planes of hexagonal carbon atomsthat constitute the particles of the original or starting graphiticmaterial have been chemically modified, converted, re-arranged,re-oriented, linked or cross-linked, merged and integrated, andhalogenated.

Example 1: Preparation of Discrete Oxidized Nano Graphene Platelets(NGPs) or GO Sheets

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, 500 grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight.

The reverse roll transfer procedure was then followed to make theresulting suspension into thin films and thick films of GO (10 nm, 100nm, 1-25 μm, 100 μm, and 500 μm in thickness), on a polyethyleneterephthalate (PET) film. For comparison purposes, GO layers of acomparable thickness range were also prepared by drop-casting and Commacoating techniques.

For making halogenated graphene films, several GO films were subjectedto aging and halogenating treatments that typically involve an agingtemperature of 30-100° C. for 1-8 hours, followed by halogenatingtreatment at 25-250° C. for 1-24 hours.

Example 2: Preparation of Single-Layer Graphene Oxide Sheets fromMeso-Carbon Micro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulphate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. The suspension wasthen coated onto a PET polymer surface using a reverse roll transfercoating and separately, a comma coating procedure to form oriented GOfilms. The resulting GO films, after removal of liquid, have a thicknessthat can be varied from approximately 0.5 to 500 μm. Halogenationtreatments were conducted before (using HF acid) and after thedispensing step (e.g. F₂ and Br₂ plasma, further discussed later).

Example 3: Preparation of Graphene Oxide (GO) Suspension and GO Gel fromNatural Graphite

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

For comparison purposes, we also have prepared GO gel samples byextending the oxidation times to approximately 96 hours. With continuedheavy oxidation, the dark-colored, opaque suspension obtained with 48hours of oxidation turns into a brown-yellowish solution that istranslucent upon rinsing with some water.

By dispensing and coating the GO suspension or the GO gel on a PET film,using both reverse roll coating and slot-die coating, and removing theliquid medium from the coated film we obtained a thin film of driedgraphene oxide. GO films were then subjected to different heat andhalogenation treatments. The heat treatments typically include an agingtreatment at 45° C. to 150° C. for 1-10 hours. The halogenationtreatments are discussed in Examples 4 and 5.

Scanning electron microscopy (SEM), transmission electron microscopy(TEM) pictures of lattice imaging of the graphene layer, as well asselected-area electron diffraction (SAD), bright field (BF), anddark-field (DF) images were also conducted to characterize the structureof integrated layer of halogenated graphene materials. For measurementof cross-sectional views of the film, the sample was buried in a polymermatrix, sliced using an ultra-microtome, and etched with Ar plasma.

A close scrutiny and comparison of FIG. 6(A), FIG. 6(A), and FIG. 7(B)indicates that the graphene layers in a halogenated GO film hereinproduced are substantially oriented parallel to one another; but this isnot the case for flexible graphite foils and GO or GH paper. Theinclination angles between two identifiable layers in the integratedfilm of halogenated graphene are mostly less than 5 degrees. Incontrast, there are so many folded graphite flakes, kinks, andmis-orientations in flexible graphite that many of the angles betweentwo graphite flakes are greater than 10 degrees, some as high as 45degrees (FIG. 6(B)). Although not nearly as bad, the mis-orientationsbetween graphene platelets in graphene paper (FIG. 7(B)) are also high(average >>10-20°) and there are many gaps between platelets. Theintegrated halogenated graphene film is essentially gap-free.

Example 4: Halogenating Treatments of GO after Deposition of a GO Layer

Chlorination of GO platelets was conducted with chloroform (CF) and,separately, with chlorobenzene (CB) at a temperature of 50-100° C. for1-10 hours. The extent of chlorination of GO was evaluated by Ramanspectroscopy and X-ray photoelectron spectroscopy (XPS). In order todetermine the effect of CF or CB treatment on the dielectric performanceof the resulting chlorinated GO, films of approximately 70 nm to 2 μm inthickness were prepared.

The fluorination of reduced graphene oxide sheets (mono- andmultilayered) can be conducted in plasmas containing CF₄, SF₆, XeF₂,fluoropolymers, or Ar/F₂ as fluorinating agents. The fluorine content ofthe resulting fluorinated graphene can be varied by changing the plasmatreatment time as well as the fluorinating agent type.

A number of techniques were used to fluorinate graphene oxide, includingexposure to F₂ gas at moderate temperatures (400-600° C.) and treatmentwith F-based plasmas. XeF₂ is a strong fluorinating agent for grapheneoxide without etching, thereby providing a facile route for graphenehalogenation. Characterization of this process via X-ray photoelectronspectroscopy (XPS) and Raman spectroscopy reveals room-temperaturefluorination saturates 25-50% coverage (corresponding to a formulaC₄F—C₂F) for single-sided exposure and CF for double-sided exposure. Dueto its high electronegativity, fluorine induces strong chemical shiftsin the carbon is binding energy allowing the use of X-ray photoelectronspectroscopy (XPS) to quantify composition and bonding type.

Fluorination was also conducted in a plasma-enhanced chemical vapordeposition (PECVD). In a typical procedure, a PECVD chamber wasevacuated to approximately 5 mTorr, and the temperature was increasedfrom room temperature to 200° C. The CF₄ gas was then introduced intothe chamber at controlled gas flow rates and pressures. The degree offluorination of the GO sample was adjusted by varying the exposure time.A suitable CF₄ plasma exposure time was found to be from 3 to 7 minutesper nm of Go layer thickness. For instance, for a GO layer 10 nm thick,the exposure time was 30 to 70 minutes.

Bromination and iodination of Go was conducted by similar proceduresusing Br₂, I₂, BrI, CBr₄, and/or CCl₄ gas or plasma under comparableconditions.

Example 5: Preparation of Various Halogenated Graphene OxideSheets/Molecules Prior to the Dispensing and Deposition Step

Graphene fluoride suspensions were obtained by chemically etchinggraphite fluoride particles (commercially available) via sonication inthe presence of sulfolane, dimethyl-formamide (DMF), orN-methyl-2-pyrrolidone (NMP). In this process, the solvent moleculesintercalate within the inter-graphene layers, weakening the van derWaals forces between neighboring layers and facilitating the exfoliationof graphite fluoride into graphene fluoride suspension. The GFsuspension could be directly coated onto the surface of a PET film usingcomma coating, slot-die coating, or, preferably, reverse roll transfercoating under a high shear condition (e.g. a higher line speed ratio,2/1 to 5/1).

Graphene fluoride with different fluorine contents could be readilyobtained by the chemical reaction of graphene oxide with hydrofluoricacid. Fluorination of graphene oxide can be done by exposing grapheneoxide to anhydrous HF vapors at various temperatures or photochemicallyat room temperature using HF solution. These procedures were conductedon GO sheets or molecules prior to the step of dispensing anddepositing.

Both single-layer and few-layer graphene can be chlorinated up to 56-74wt. % by irradiation with UV light in a liquid chlorine medium. Thebromination of both single-layer and few-layer graphene can be conductedunder comparable conditions.

As examples, GO (15 mg) was mixed in 30 mL of carbon tetrachloride andsonicated for 20 min using a tip-style ultrasonicator. The suspensionobtained was transferred to a 500 mL quartz vessel which was fitted witha condenser maintained at 277° C. The reaction chamber was purged withhigh purity nitrogen for 30 min and chlorine gas was passed through thechamber. The gaseous chlorine condensed in the quartz vessel. The quartzvessel containing around 20 mL of liquid chlorine was heated to 250° C.with simultaneous irradiation of UV light (250 Watt high pressure Hgvapor lamp) for 1.5 h. The solvent and excess chlorine was removed,leaving a transparent film on the walls of the quartz vessel. The solidwas dispersed in absolute alcohol under ultrasonication, filtered andwashed with distilled water and absolute alcohol. The filtrate was thenre-dispersed in 40 ml of distilled water, ultrasonicated for 2 min andcentrifuged. The black supernatant obtained was separated and filteredusing a PVDF membrane (200 nm pore size). The yield from the 15 mg GOsample was around 7-10 mg. The chlorinated graphene sheets can bedispersed in a solvent (e.g. CCl₄) to form a suspension for dispensingand depositing.

In the case of bromination, 12 mg of graphene was mixed in the quartzvessel, to which 20 mL of liquid bromine was added. The mixture wassonicated for 10 min using an ultrasonicator. To this 0.5 g of carbontetrabromide was added. The quartz vessel was then heated to 250° C.with simultaneous irradiation of UV light just as in chlorination. Theexcess bromine was removed and the product washed with sodiumthiosulfate. The solid residue was then washed with water and absolutealcohol several times to remove sodium thiosulfate and then dispersed in40 mL distilled water and centrifuged. The black supernatant obtainedwas separated and filtered using a PVDF membrane (200 nm pore size). Theyield from the 12 mg sample was around 5-7 mg. The brominated graphenesheets can be dispersed in a solvent (e.g. CBr₄) to form a suspensionfor dispensing and depositing.

A desired amount of halogenated GO sheets/molecules can be added to a GOsuspension or GO gel to produce a GO/halogenated GO mixture suspensionor gel prior to the dispensing/depositing step. Typically, a halogenatedGO-to-GO weight ratio is from 10/1 to 1/10 (in selected liquid medium,such as DMF and NMP), more typically from 1/1 to 1/10 (if the liquidmedium is water).

Example 6: Properties of Integrated Layers of Halogenated Graphene

The measurement methods for dielectric strength, dielectric constant,volumetric resistivity (reciprocal of electric conductivity) arewell-known in the art. Standardized methods were followed in the presentstudy: dielectric strength (ASTM D-149-91), dielectric constant (ASTMD-150-92), and volume resistivity (ASTM D-257-91).

FIG. 8 shows the dielectric breakdown strength of GO film, presentlyinvented GO-derived integrated graphene fluoride film (prepared by thereverse roll transfer coating process), and prior art polyimide filmplotted as a function of the film thickness. These data indicate thatthe presently invented halogenated graphene films exhibit anexceptionally high dielectric breakdown strength (>12 MV/cm) even with afilm thickness as high as 25-125 μm (unusual and unexpected). Thedielectric breakdown strength values were found to be relativelyindependent of the film thickness. By contrast, for the same thicknessrange, the graphene oxide films endure a dielectric strength of 0.62-1.1MV/cm, one order of magnitude lower. For comparison purposes, thecommercially available polyimide films (du Pont Kapton films) have adielectric strength of 1.54-3.03 MV/cm.

The dielectric breakdown strength of GO-derived fluorinated graphenefilms and chlorinated graphene films (both prepared by a reverse rolltransfer procedure and, separately, by a casting procedure) are plottedas a function of the degree of fluorination (atomic ration, F/(F+O) orthe degree of chlorination (atomic ratio, Cl/(Cl+O) in FIG. 9. Thecasting procedure did not involve any significant amount of shearstress, but the reverseroll coating process includes high shear stressesin orienting GO and halogenated GO films. These data demonstrate that,given the same 100 nm thickness, the dielectric strength of the GO filmswere increased from 2.6 MV/cm for a pure GO film (no fluorination orzero degree of fluorination) to 22.2 MV/cm for a fully fluorinated film.This again asserts that fluorination can impart dramatically improveddielectric strength to the GO films. This is unexpected. Integratedfilms of chlorinated GO films follow the same trend.

Most significantly and also unexpectedly, the shear induced orientationof fluorinated or un-fluorinated GO sheets or molecules enables theintegrated film to endure a significantly higher dielectric breakdownstrength (2.6-22.2 MV/cm) than those (1.1-7.2 MV/cm) of the un-orientedor less oriented counterparts prepared by conventional casting.Similarly, integrated films of highly oriented chlorinated GO sheets ormolecules also deliver much higher dielectric strength as compared totheir cast counterparts wherein the sheets/molecules are not properlyoriented. This is a highly significant discovery, which provides aversatile strategy for achieving exceptional dielectric strength ofgraphene materials.

Summarized in FIG. 10 are dielectric breakdown strength of GO-derivedfluorinated graphene films (prepared by a reverse roll transferprocedure) and GO-derived fluorinated graphene paper prepared by theconventional paper-making procedure (vacuum-assisted filtration) plottedas a function of the degree of fluorination in terms of the atomicratio, F/(F+O). These data show that the dielectric strengths (1.1-1.45MV/cm) of GO-based paper membranes prepared by the conventional methodof vacuum-assisted filtration of multiple fluorinated GO sheets arerelatively poor and are relatively independent of the degree offluorination. Although some degree of orientation is achieved with thismethod, the dielectric strength remains very low. It seems that thedefects, voids, kinks, and disruption of fluorinated GO sheets are localsites of concentrated electric fields that initiated the dielectricbreakdown in these imperfections at a relatively low global (average)low voltage levels and then rapidly propagated across the entire sample.The presently invented process eliminates these imperfections.

Summarized in FIG. 11 are the dielectric constants of GO-derivedfluorinated graphene films and brominated graphene films plotted as afunction of the degree of fluorination (atomic ration, F/(F+O) or thedegree of bromination (atomic ratio, Br/(Br+O). As the degree offluorination or bromination increases, the dielectric constants of theintegrated film of halogenated graphene increases, reaches a maximum,and then begins to decrease after the degree of fluorination orbromination exceeds 0.6. Most noteworthy is the observation that thepartially halogenated GO films (CsZA, wherein Z is a halogen elementselected from F, Cl, Br, I, x=0.01 to 6.0, y=0 to 5.0, and x+y≤6.0) canexhibit a dielectric constant from 3.9 to 22.2, in contrast to thedielectric constant value of 2.3 for integrated layer of GO (x=0;C₆O_(y))

In summary, the integrated films of halogenated graphene (highlyoriented GO-derived graphene halide films, GOGH) prepared from originalgraphene oxide suspension or GO gel using an orientation-controllingshear stress-based process have the following characteristics:

-   -   (1) The integrated halogenated graphene films (thin or thick)        are an integrated halogenated graphene oxide or essentially        oxygen-free graphene halide structure that is typically a        poly-crystal having large grains. The film has wide or long        chemically bonded graphene planes that are all essentially        oriented parallel to one another. In other words, the        crystallographic c-axis directions of all the constituent        graphene planes in all grains are essentially pointing in the        same direction.    -   (2) The reverse roll coating is very effective in achieving a        high degree of graphene plane orientation and graphene halide        crystal perfection.    -   (3) The co-existence of halogen and oxygen in the integrated        halogenated GO layers leads to unexpected synergistic effect in        producing films of high dielectric constants.    -   (4) The GOGH film is a fully integrated, essentially void-free,        single graphene entity or monolith containing no discernable        discrete flakes or platelets that were previously present in the        original GO suspension. In contrast, the paper or membrane of        graphene halide or GO platelets (each platelet <100 nm) are a        simple, un-bonded aggregate/stack of multiple discrete platelets        of GO or halogenated GO. The platelets in these paper/membranes        are poorly oriented and have lots of kinks, bends, and wrinkles.        Many voids or other defects are present in these paper/membrane        structures, leading to poor dielectric breakdown strength.    -   (5) In prior art processes, discrete graphene or GO sheets        (<<100 nm, typically <10 nm) that constitute the original        structure of graphite particles could be obtained via expanding,        exfoliating, and separating treatments. By simply mixing and        re-compressing these discrete sheets/flakes into a bulk object,        one could attempt to orient these sheets/flakes hopefully along        one direction through compression. However, with these        conventional processes, the constituent flakes or sheets of the        resulting aggregate would remain as discrete        flakes/sheets/platelets that can be easily discerned or clearly        observed even with an un-assisted eye or under a        low-magnification optical microscope (×100-×1000).

In contrast, the preparation of the presently invented integrated filmsof halogenated graphene involves heavily oxidizing the original graphiteparticles, to the extent that practically every one of the originalgraphene planes has been oxidized and isolated from one another tobecome individual graphene planes or molecules that possess highlyreactive functional groups (e.g. —OH, >O, and —COOH) at the edge and ongraphene plane surfaces. These individual hydrocarbon molecules(containing elements such as O and H, in addition to carbon atoms) aredispersed in a liquid medium (e.g. mixture of water and alcohol) to forma GO dispersion. This dispersion is then reverse roll-coated onto asmooth substrate surface, and the liquid components are then removed toform a dried GO layer. When slightly heated or aged, these highlyreactive molecules react and chemically join with one another mostly inlateral directions along graphene planes (in an edge-to-edge manner toincrease the length and width) and, in some cases, between grapheneplanes as well.

Illustrated in FIG. 7(D) is a plausible chemical linking mechanism whereonly 2 aligned GO molecules are shown as an example, although a largenumber of GO molecules can be chemically linked together to form a film.Further, chemical linking could also occur face-to-face, not justedge-to-edge. These linking and merging reactions proceed in such amanner that the molecules are chemically merged, linked, and integratedinto one single entity. The molecules or “sheets” become dramaticallylonger and wider. The molecules (GO sheets) completely lose their ownoriginal identity and they no longer are discrete sheets/platelets.There is only one single layer-like structure that is essentially anetwork of interconnected giant molecules with an essentially infinitemolecular weight. This may also be described as a graphene poly-crystal(with several grains, but typically no discernible, well-defined grainboundaries). All the constituent graphene planes are very large inlateral dimensions (length and width) and are aligned parallel to oneanother.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the graphitic film is composed of several huge grapheneplanes (with length/width typically >>100 μm, more typically >>1 mm).These giant graphene planes are stacked and bonded along the thicknessdirection (crystallographic c-axis direction) often through not just thevan der Waals forces (as in conventional graphite crystallites), butalso covalent bonds. In these cases, wishing not to be limited bytheory, but Raman and FTIR spectroscopy studies appear to indicate theco-existence of sp² (dominating) and sp³ (weak but existing) electronicconfigurations, not just the conventional sp² in graphite.

-   -   (6) This integrated GOGH film is not made by gluing or bonding        discrete flakes/platelets together with a resin binder, linker,        or adhesive. Instead, GO or halogenated GO sheets (molecules) in        the dispersion or gel are merged through joining or forming of        covalent bonds with one another, into an integrated graphene        entity, without using any externally added linker or binder        molecules or polymers. These GO or halogenated GO molecules are        “living” molecules capable of linking with one another in a way        similar to living polymers chains undergoing “recombination”        (e.g. a living chain of 1,000 monomer units and another living        chain of 2,000 monomer units combine or join to become a polymer        chain of 3,000 units). A 3,000-unit chain can combine with a        4,000-unit chain to become a giant chain of 7,000 units, and so        on.    -   (7) This integrated film is typically a poly-crystal composed of        large grains having incomplete grain boundaries, typically with        the crystallographic c-axis in all grains being essentially        parallel to each other. This entity is derived from a GO        suspension or GO gel, which is in turn obtained from natural        graphite or artificial graphite particles originally having        multiple graphite crystallites. Prior to being chemically        oxidized, these starting graphite crystallites have an initial        length (L_(a) in the crystallographic a-axis direction), initial        width (L_(b) in the b-axis direction), and thickness (L_(c) in        the c-axis direction). Upon heavy oxidation, these initially        discrete graphite particles are chemically transformed into        highly aromatic graphene oxide molecules having a significant        concentration of edge- or surface-borne functional groups (e.g.        —OH, —COOH, etc.). These aromatic, halogenated GO molecules in        the GO suspension have lost their original identity of being        part of a graphite particle or flake. Upon removal of the liquid        component from the suspension and after thermal aging, the        resulting GO molecules are chemically merged and linked into a        unitary or monolithic graphene entity that is highly ordered.

The resulting unitary graphene entity typically has a length or widthsignificantly greater than the L_(a) and L_(b) of the originalcrystallites. The length/width of this graphitic film is significantlygreater than the L_(a) and L_(b) of the original crystallites. Even theindividual grains in a poly-crystalline graphitic film have a length orwidth significantly greater than the L_(a) and L_(b) of the originalcrystallites.

-   -   (8) Due to these unique chemical compositions (including oxygen        content), morphology, crystal structure (including        inter-graphene spacing), and structural features (e.g. high        degree of orientations, few defects, chemical bonding and no gap        between graphene sheets, and no interruptions in graphene        planes), the highly oriented graphene oxide-derived halogenated        GO film has a unique combination of outstanding dielectric        constants and dielectric breakdown strength.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of dielectric materials:monolithic integrated films of highly oriented graphene halide. Thechemical composition (oxygen and halogen contents), structure (crystalperfection, grain size, defect population, etc.), crystal orientation,achievable thickness with a high degree of orientation, morphology,process of production, and properties of this new class of materials arefundamentally different and patently distinct from any known graphenematerials. These halogenated graphene films can be used as a dielectricmaterial component in a wide variety of microelectronic devices.

We claim:
 1. A process for producing an integrated layer of highlyoriented halogenated graphene sheets or molecules, said processcomprising: (a) preparing either a graphene oxide dispersion havinggraphene oxide sheets dispersed in a fluid medium or a graphene oxidegel having graphene oxide molecules dissolved in a fluid medium, whereinsaid graphene oxide sheets or graphene oxide molecules contain an oxygenamount 5% by weight to 46% by weight; (b) introducing a halogenatingagent into said graphene oxide dispersion or gel and effecting achemical reaction between said halogenating agent and said grapheneoxide sheets or molecules to form a dispersion of halogenated graphenesheets or a gel of halogenated graphene molecules, wherein saidhalogenated graphene sheets or molecules have a chemical formula ofC₆Z_(x)O_(y), wherein Z is a halogen element selected from F, Cl, Br, I,or a combination thereof, x=0.01 to 6.0, y=0 to 5.0, and x+y<6.0; (c)dispensing and depositing a layer of said halogenated graphenedispersion or gel onto a surface of a supporting substrate under a shearstress condition, wherein said dispensing and depositing procedureincludes shear-induced thinning of said halogenated graphene dispersionor gel and shear-induced orientation of halogenated graphene sheets ormolecules, to form a wet layer of halogenated graphene on saidsupporting substrate; and (d) removing said fluid medium from the wetlayer of halogenated graphene to form said integrated layer ofhalogenated graphene having an inter-planar spacing d₀₀₂ of 0.35 nm to1.2 nm as determined by X-ray diffraction.
 2. The process of claim 1,wherein the graphene oxide sheets contain single-layer graphene oxide orfew-layer graphene oxide sheets each having 2-10 oxidized grapheneplanes.
 3. The process of claim 1, wherein the dispensing and depositingstep includes a reverse roll transfer coating procedure.
 4. The processof claim 1, wherein the dispensing and depositing step includes a slotdie coating or comma coating procedure.
 5. The process of claim 1,further comprising a step (e) of heat treating the integrated layer oforiented halogenated graphene at a first heat treatment temperaturehigher than 100° C. but no greater than 3,200° C. for a desired lengthof time to produce a graphitic film having an inter-planar spacing d₀₀₂less than 0.4 nm and a combined oxygen/halogen content less than 1% byweight.
 6. The process of claim 1, wherein the fluid medium consists ofwater, an alcohol, a mixture of water and alcohol, or an organicsolvent.
 7. The process of claim 1, further comprising a compressionstep, during or after removing the fluid medium, to reduce a thicknessof said integrated layer.
 8. The process of claim 1, wherein thegraphene oxide dispersion or gel has a graphene oxide content of 1% to50% by weight prior to introducing the halogenating agent.
 9. Theprocess of claim 1, wherein the graphene oxide dispersion or gel has agraphene oxide content of 3% to 50% by weight prior to introduction thehalogenating agent, and the graphene oxide dispersion or gel possesses aliquid crystal phase.
 10. The process of claim 1, wherein the step ofremoving the fluid medium from the wet layer of halogenated graphene iscarried out by heat treatment, vacuum, or a combination thereof.
 11. Theprocess of claim 1, wherein the step of forming a dispersion ofhalogenated graphene sheets or a gel of halogenated graphene moleculescomprises irradiation with UV light in a liquid medium containingchlorine or bromine.
 12. The process of claim 1, wherein the integratedlayer of halogenated graphene has a thickness from 10 nm to 500 μm.